1. Field of the Invention
The present invention relates to a curl correcting or decurling device for correcting the curl of a rolled sheet member.
2. Related Background Art
Presently popular office equipment such as facsimile apparatus and printers generally employ a long recording sheet rolled on a core in their recording system.
Because of the rolled state, such a sheet tends to show curling, and such a curled sheet often ends in jamming up the transportation. Therefore, for the purpose of eliminating or reducing such curl, a curl correcting mechanism is often employed in the recording apparatus utilizing such rolled sheets.
FIG. 29 shows an example of such a curl correcting mechanism, in which, in a main body 51 and a cover member 52 openably linked with a pin 53, a roll holder 54, a platen roller 55 and a cutter 56 are provided in the main body 51 while a curl correcting shaft 57 constituting the curl correcting mechanism and a recording head 58 are fixed on the cover member 52.
In the above-explained mechanism, a roll 59a of a sheet material 59 is loaded in the roll holder 54, and the sheet material 59 is advanced through the shaft 57 by the rotation of the platen roller 55, subjected to image formation by the recording head 58 and discharged from the mechanism after cutting by the cutter 56.
In this mechanism, the curl is reduced by bending the sheet material 59 by the curl correcting shaft 57 in a direction opposite to that of curling. In such a mechanism, the curl correcting effect is known to become larger as the wrapping angle .alpha., shown in FIG. 30, of the sheet material 59 on the curl correcting shaft 57 becomes larger.
In the curl correcting mechanism shown in FIG. 29, the diameter of the sheet roll 59a decreases as the sheet material 59 is used, whereby the wrapping angle of the sheet material 59 on the shaft 57 gradually decreases from .alpha..sub.1 to .alpha..sub.2. Consequently, the curl correcting effect on the sheet material 59 decreases gradually, and satisfactory curl correction is often not achieved when the diameter of the sheet roll 59a is small where the curling tendency is stronger. FIG. 31 shows another curl correcting mechanism further having a guide shaft 60 for maintaining a constant wrapping angle .alpha. of the sheet material 59 on the curl correcting shaft 57. The platen roller 55 and the guide shaft 60 are mounted in the main body 51 while the shaft 57 and the recording head 58 are mounted on the cover member
In this mechanism, the shaft 57 and the recording head 58 are rotated upwards together with the cover member 52 about the pin 53 to open the upper face of the main body 51 as shown by chain lines, whereby the loading of the sheet material 59 into the main body 51 is facilitated.
In the curl correcting mechanism shown in FIG. 31, the guide shaft 60 of the main body cannot be positioned above the shaft 57, as the shaft 57 rotates together with the cover member 52. For this reason, the wrapping angle .alpha. of the sheet material 59 on the shaft 57 can only be increased up to about 90.degree., so that a satisfactory curl correcting effect cannot be obtained.
For increasing the curl correcting effect, the wrapping angle .alpha. has to be further increased, for example by positioning the guide shaft 60 in the rotating direction of the curl correcting shaft 57 as shown in FIG. 32A, but, in such an arrangement, the cover member 52 cannot be rotated upwards at the loading of the sheet material 59 into the main body 51 as the shaft 57 interferes with the guide shaft 60.
For this reason the shaft 57 and the guide shaft 60 have to be mounted in the main body 51. However, at the loading of the sheet material 59, it then becomes necessary to thread the leading end of the sheet material 59 between the shafts 57 and 60 and pass it around the shaft 57, so that the loading operation becomes cumbersome.
Also for mounting the shaft 57 on the cover member 52 and still obtaining a large wrapping angle .alpha. of the sheet material 59 on the shaft 57, it is necessary to position the shaft 57 quite some distance from the platen roller 55 and the guide shaft 60 as shown in FIG. 32B, but such an arrangement requires a significantly extended sheet path, resulting in frequent sheet jamming.
FIG. 33 shows another curl correcting mechanism, in which, in a main body 81 and a cover member 82 openably linked with a pin 83, a roller holder 84, a platen roller 85, a cutter 86 and a guide member 87a constituting the curl correcting mechanism are mounted in the main body 81 while a recording head 88, and a guide member 87b constituting the curl correcting mechanism in cooperation with said guide member 87a are mounted on the cover member 82.
In this mechanism, sheet material 89, formed as a roll 89a, is advanced by the rotation of the platen roller 85, subjected to the recording head 88 and discharged from the mechanism after cutting by the cutter 86. The curl of the sheet material 89 is reduced when it passes a bent path formed by the guide members 87a, 87b and is bent in a direction opposite to that of curling.
Furthermore there has been proposed another curl correcting mechanism as shown in FIG. 34. In this mechanism, arms 91, rotatably mounted on both ends of a guide shaft 90, rotatably supports a curl correcting shaft 92 at their free ends. Said arms 91 are constantly biased counterclockwise by a spring but the driven clockwise through a friction clutch (not shown) when the platen roller 85 is rotated in a direction indicated by the arrow. Stoppers 93, 94 are provided in predetermined positions of the main body 81 for limiting the rotation of the arms 91.
In the above-explained mechanism, the sheet material 89 is advanced by the rotation of the platen roller 85 in the direction indicated by the arrow. At the same time, the arms 91 rotate clockwise but are stopped by contact with the stopper 94 whereupon the friction clutch starts to slip. Thus the advancing sheet material 89 is guided through the guide shaft 90 from the roll 89a and is bent by the shaft 92 in a direction opposite to that of the curl, whereby the curl is reduced.
When the rotation of the platen roller 85 is terminated, the arms 91 return to the stand-by position in contact with the stopper 93, by the biasing force of the spring.
The curling of the sheet material 89 depends not only on the roller diameter but also on other factors such as the thickness and kind of the sheet material 89, and the time that has passed since the sheet is rolled. For example, thermal paper has a thickness of about 65 .mu.m while plain paper used for ink transfer recording has a thickness of about 85 .mu.m, so that the plain paper tends to show stronger curling for a given roll diameter.
However, in the curl correcting mechanism shown in FIG. 33, the bending angle defined by the curl correcting guide members 87a, 87b is constant.
Also in the curl correcting mechanism shown in FIG. 34, the curl correcting effect determined by the position of the arms 91 or curl correcting shaft 92 is kept constant by the stopper 94.
Consequently, if the bending angle defined by the guide members 87a, 87b or the position of the stopper 94 is designed to correct curl in a thermal recording sheet, the curl correcting effect becomes insufficient for the plain recording sheet. On the other hand, if the curl correcting conditions are determined for the plain recording paper, an excessive correcting effect will appear for the thermal recording sheet, which will therefore be curled in the opposite direction. In either case there will be defective sheet transportation, leading eventually to sheet jamming.
For example, the recording unit of a facsimile apparatus is designed to use both thermal recording paper and plain recording paper, and appropriate curl correction cannot be attained depending on the thickness or the kind of the sheet material 89, if the curl correcting effect is fixed as it is in the devices shown in FIGS. 33 or 34.